The present invention is employed in electrophotographic printers where laser scan lines emanating from a multiple beam light source are projected onto a photoconductive surface. In the case of laser printers, facsimile machines, and the like, it is common to employ a raster output scanner (ROS) as a source of energy signals to be imaged on a pre-charged photoreceptor (a photosensitive plate, belt, or drum) for purposes of xerographic printing. The ROS provides at least two laser beams that switch on and off or are modulated in intensity in response to image signals associated with the desired image to be printed as the beams are made to scan across the photoreceptor surface. In the process called charged area development (CAD), the photoreceptor surface is selectively rasterwise discharged by interlaced scan lines in locations to be printed white, to form the desired image on the photoreceptor. In the inverse process known as discharged area development (DAD), locations to be pigmented are selectively discharged instead. The modulation of the laser beams creating the desired latent image on the photoreceptor is provided by digital electronic data signals controlling drivers or modulators associated with the multi-beam laser source. A common technique for scanning the beams across the photoreceptor is to employ a rotating polygon mirror or similar reflective device; the beams emitted from the laser sources are reflected by the mirror facets creating a scanning action that forms interlaced scan lines, or a raster, on the moving photoreceptor surface. Once a latent image is formed on the photoreceptor, it is subsequently developed with marking particles and the developed image is transferred to a copy sheet, as in the well-known process of xerography.
A known rotating polygon multi-beam ROS scanner system is described below, for easier understanding of the technical discussion, in Prior Art FIG. 7. FIG. 7 shows a pair of sagittally offset laser diodes 31 and 32. The beams 41 and 42 emitted by laser diodes 31 and 32 are collimated by collimator 33 which may be a compound design but is shown schematically as the single lens L1 in FIG. 7. A sagittal aperture 34 following the collimator controls the numerical aperture of the collimator, which in turn controls the spot size created by the beams on the photoreceptor. The input cylinder optical element 35 (lens L2) focuses the beams 41 and 42 on the surface of a polygon facet 36 of the rotating polygon. After reflecting from facet 36, beams 41 and 42 pass through the f-.THETA. lens 37 (lens L3). The main function of the f-.THETA. lens 37 is to provide focusing in the tangential meridian and control the scan linearity, in terms of uniform spot displacement per unit angle of polygon rotation.
Subsequently, the wobble compensating optical element (WCO) 39 reimages the focused beams 41 and 42 reflected from polygon facet 36 onto the photoreceptor (PR) plane 40 at a predetermined position, independently of the polygon angle error or tilt of the current facet 36. The WCO can consist of a toroidal surface in the F-.THETA. lens, a postpolygon cylinder mirror or a postpolygon cylinder lens. Such compensation is possible because the focused beams are stationary "objects" for the F-.THETA. lens 37 and the WCO 39. Although, due to polygon tilt, or wobble, the beams 41 and 42 reflecting from the mirror facets are directed to different positions of the postpolygon optics aperture for each different facet of the rotating polygon, to a good approximation beams 41 and 42 are imaged to the same position on the PR plane 40. It should be appreciated that in Prior Art FIG. 7, the chief rays are not telecentric. That is, the chief exit rays are not parallel with the system axis 38.
Heretofore, a number of patents and publications have disclosed multiple beam, ROS-based recording systems, the relevant portions of which may be briefly summarized as follows:
U.S. Pat. No. 4,390,235 to Minoura, issued Jun. 28, 1983, discloses a multi-beam scanning apparatus for scanning a surface with a plurality of beam spots modulated independently of one another. Included in the system is an anamorphic afocal zoom lens which has the function of changing the angular magnification, resulting in a proportional change in the spot size as well.
U.S. Pat. No. 4,474,422 to Kitamura, issued Oct. 2, 1984, teaches a multibeam optical scanning apparatus employing a collimating portion positioned subsequent to a polygon reflector.
U.S. Pat. No. 5,257,048 to Genovese, issued Oct. 26, 1993, discloses a ROS-based imaging system including a specially designed optical element to facilitate the precise registration of light beams on the surface of a photoreceptor. Similarly, in U.S. Pat. No. 5,300,962, issued Apr. 5, 1994, (and as published in the Xerox Disclosure Journal, Vol. 18, No. 1, pp. 87-93, Jan./Feb. 1993) Genovese discloses a multi-beam ROS providing a plurality of independently controllable, substantially parallel writing beams.
In accordance with the present invention, there is provided a raster output scanner for a printing apparatus, comprising:
One aspect of the present invention deals with a basic problem of scanline bow in multiple-beam ROSs, and more particularly differential scanline bow as is commonly found in multiple color printing systems. As used herein, the term scanline bow refers to the deviation from parallel of a pair or plurality of scanlines imaged on the surface of a photoreceptor. This aspect is further based on the recognition that pre-scanning telecentricity alleviates this problem or, more specifically, that multiple beams parallel with a chief optical ray strike the reflecting facet at normal incidence and avoid the creation of hyperbolic beam scan paths as is common in may ROS optical systems. The present technique for alleviating differential scanline bow is directed toward achieving perpendicularity at the scanning surface (i.e., the polygon facet) so as to reduce the scanline bow at the photoreceptor image plane while simultaneously providing a physically shorter prepolygon optical train. The technique described herein is advantageous because it is relatively simple, and inexpensive compared to other approaches designed to eliminate or compensate for differential scanline bow. Moreover, elimination of differential scanline bow allows more beams to be scanned simultaneously thereby enabling higher processing speeds and/or higher resolutions in ROS imaging systems.